The global positioning system (GPS) is a satellite-based system that is relied upon to provide accurate positioning information to GPS-enabled receivers, especially for military aircraft and weapon systems applications and for civilian aviation. Inaccurate positioning information can result in less accurate GPS-guided weapons which may increase collateral damage as well as a need for an increased number of weapons needed to accomplish a given mission. Additionally, civilian aviation applications rely on GPS positioning information to be highly accurate for aircraft navigation.
A next generation or version of GPS that is being developed is GPS III which has a higher accuracy requirement, yet includes design considerations that may affect this higher accuracy requirement. In particular, GPS III is implemented with a steerable spot beam antenna that is used to transmit high-intensity spot beams of GPS signals. However, the high gain of a spot beam antenna is obtained at the cost of reduced beamwidth, and precision pointing of the narrow beam is needed for the higher accuracy requirements of the GPS III system. As such, the gyros that control the attitude of the GPS satellite need to be calibrated for gyro drift errors.
Gyro measurements are not absolute, but rather relative to some frame of reference. For typical satellite gyros, this frame of reference is a computed inertial frame, where the orientation drifts from a true inertial frame due to various sources such as measurement noise, scale factor error, gyro bias error, and others. In order to provide accurate attitude and rate estimates, the gyros need to be calibrated frequently with a reference attitude sensor.